Occultation of iota Cancri by (411) Xanthe Tues. evening, April 17/18 - New 2007 April 16, 23h UT
This is the brightest star occulted by a sizeable asteroid in North America this year
But the latest weather forecast for northeastern Florida, the best place for the event, is "iffy"
Please watch 4.2-mag. iota Cancri blink and time its occultation by (411) Xanthe, in s.w. Georgia and n.e. Florida at 8:56 pm EDT Tues. evening, April 17, to help IOTA measure the size and shape of the asteroid, estimated to be 76 km in diameter. Please let us know your plans so that mobile observers don't duplicate your observation. We need as many observers as possible to try to observe this occultation of the brightest star by a medium-sized asteroid to occur in North America this year. At least 4 observers will be mobile for this and they want to fill in gaps of coverage by fixed-site observers to best determine the shape of Xanthe. Such a bright star should be easy to observe with binoculars, or even naked eye; if you're observing visually, you don't need to bother to set up a telescope. For methods of timing occultations, click here. The 76-km-wide path extends from northwest to southeast, passing over the s.w. corner of Georgia, and then sweeping across n.e. Florida. Jacksonville is north of the predicted path, but within the "1-sigma" uncertainty zone, so an occultation could occur there. Similarly, Gainesville is at the predicted southern limit, so observers there also have a good chance for an event. Observers as much as ten times the path-width away from the center for the occultation of iota Cancri by (411) Xanthe Tues. evening have a chance to observer an occultation by a satellite of Xanthe. The asteroid is not known to have a satellite, but it could have one, so observers far from the path are encouraged to at least watch the star with binoculars from 2 minutes before to 2 minutes after the time of closest approach for your location. Ten times the path width means that observers throughout Florida have a chance, as well as those as far north as Raleigh, North Carolina, and throughout Tennessee, and of course across Georgia and South Carolina. Also within the 10-path-width zone is Puerto Rico, where closest approach will be at 1:00 UT. With such a bright star, there should be little doubt if even a short occultation occurs or not. But the best observations of a possible satellite event would be either video, or by two visual observers about a km apart for independent verification. Also, the 4.2-magnitude spectral type G5 star can be seen with telescopes in very bright twilight, and possibly even daylight, if conditions are good. Observers with telescopes with setting circles might first find a star like Pollux to center the field and determine any offsets needed, to then move to iota Cancri. Another possibility would be to pre-point the telescope to a star of similar declination the night before, and keep the telescope clamped until the observation. We are working on producing a list of pre-point stars that can be used the night before, and will make that available as soon as we have it. Details can be found from the Web sites below, but approximately, the path in bright twilight & daylight is as follows: U.T. Sun central line h m alt appoximate location 0:49 +21 n. of Sacramento, CA (near Rocklin), & s. of Carson City, NV 0:50 +15 n. of Cedar City, UT 0:52 +10 near NM/Colorado border 0:53 +1 Norman, Oklahoma 0:54 -2 s. Arkansas 0:56 -6 central Mississippi The Astro-Weather-Meteo 48h prognosis, up to 0h UT of April 18 UT (that is, only 1 hour before the occulation), is now available here. It shows clear, or mostly clear, skies along the East Coast, over Florida except the northern panhandle, almost all of Georgia, and the Carolinas. It shows a clear zone across northern Alabama (Birmingham) and eastern Tenn. to Nashville. Unfortunately, it shows overcast, or mostly cloudy, skies west of there across Oklahoma to the New Mexico border, then partly cloudy across the path in most of n. NM and s. Colo. Then mainly clear across central Utah and western Nevada. In California, clouds are shown in the Sierras and around San Francisco, but clear around Sacramento. Of course, this can change and we'll be watching the updates that are made every 12 hours. AND IT HAS CHANGED, AS NOTED HERE: The new Astro-Weather-Meteo forecast (37h prognosis) is now available and it is not as good as the previous one, with at least partly cloudy skies now forecast across northern Florida, and somewhat worse conditions in other parts of the path as well. In northern Florida, I think locations near the Atlantic coast may have the best chance, but that remains to be seen; we'll need to watch the next forecasts, at midnight EDT tonight and noon tomorrow. The target star, also known as SAO 80416 or HIP 43103, is at J2000 RA 8h 46m 41.8s, Dec +28 deg. 45' 35". It has a 6.6-mag. companion, SAO 80415, 30" to the northwest that will not be occulted. Since the Sun altitude will be -14 deg. at the Atlantic coast, you will need to find the star in diminishing twilight. It's altitude above the horizon at the time of the event will be 85 deg., so finder (and maybe main) scopes with a diagonal eyepiece will help. Richard Nugent wrote on April 11: "This evening, at my house just 2 miles from downtown Houston, Texas I found the target star in my 4" Meade 30 minutes after sunset, and had my video on the star 5 minutes later. The target star is a double and was easily seen on video with the Watec 902H Ultimate. Rather than strain my neck with an 85 degree altitude star and a straight thru finder, I used manual setting circles to find the target star. Its declination is very close to that of the 4th mag star (sigma Geminorum) just 1 deg north of Pollux. With my scope on Pollux I turned the telescope east 1hour 3 minutes in right ascension using the RA setting circle. With a 20 x 25 minute FOV in the video this was a simple find." A N. American path map and finder charts of different scales are on Steve Preston's Web site. If an occultation occurs, there will be a 10-mag. drop that should be easily noticed by visual observers lasting 8 seconds for a central event. A list of some stations is below, a shortened version of the longer list on Derek Breit's Web site given above. I've indicated the locations of prospective observers from the messages that I've received so far; please let me know your plans so I can add you to an updated version of this list that I'll be sending later. I plan to arrive at Jacksonville airport at 2:40 pm and can bring an extra video recording system if a local observer could use it with their telescope to try to record the occultation. Between the distance and probability columns, I've added codes for those who have said that they will try this occultation, with * for a fixed-site visual observer, V for fixed video observer, m for mobile visual observer, and M for mobile video observer. Let me know your plans so I can add a code for your station in a later update. For example, I plan to try to observe from -35 and -40 km, but I will change that if anyone in Gainesville plans to try to observe the event from there. Observers even out to the 2-sigma limits are useful to ensure coverage of the occultation. For example, for the Fortuna occultation last week, we have a nearly 2-sigma south shift of the path. Fortunately, a fixed-site observatory at about 1.2 sigma south observed, and had an 11-second occultation, giving a good valuable observation near the actual southern limit for that event, completing the "picture" of that asteroid that was also well- observed across its predicted path. Several stations have been added using measurements from a map provided by Howard Cohen, Gainesville; their distances are approximate, but are probably accurate to within 2 km. Occultation of HIP 43103 (iota Cnc) by 411 Xanthe on 2007 Apr 18 Distance from center of occultation path - in km Proba- Location Star Sun km bility U.T. alt alt 91 15% ** Northern limit plus 1-Sigma ** h m s o o 76 22% Montgmry AL Whigham & McGraw 0 55 29 86 -9 74 23% GEORGETO GA TONY P. MURRAY 0 55 44 85 -10 60 31% JACKSONVILLE FL CRUM~DAVID A. 0 56 25 84 -13 43 * 40% Valdosta GA Martha Leake 0 56 7 85 -12 42 M 41% David Dunham mobile 38 43% *** Northern limit *** 37 M 43% David Dunham remote 33 m 44% Osceola Nat'l Forest, FL NEFAS site 31 m 47% Osceola Nat'l Forest, FL Mike Toomey 20 M 50% Paul Maley mobile 9 m 52% Stargate Obs Live Oak FL Howard Cohen 5 M 53% e. of Live Oak Francisco Reyes 0 53% **** Centre Line **** 2 m 53% Roger Venable mobile 0 M 53% Roger Venable remote -1 * 53% Lake City FL Tandy Carter -1 m 53% Houston FL Roger Curry (better somewhere else!) -3 53% FlaglerB FL E Noel 0 56 35 83 -14 -12 52% Lloyd FL Stephen Wingreen 0 56 2 85 -12 -13 * 52% Melrose & Keystone Hgts FL C Broward & B Connell -17 51% ORMOND B FL WALTER L. DOBBINS 0 56 37 83 -14 -20 M 50% Richard Nugent mobile -23 49% DaytonaBeach FL 0 56 38 83 -14 -23 * 49% n. of Gainesville FL Don Loftus -36 * 43% s. of Gainesville FL Scott McCartney -37 43% Gainesvl FL C.Siopis &S.Fisher 0 56 22 84 -13 -38 43% Tallahassee FL 0 55 59 86 -11 -38 43% *** Southern limit *** -57 32% H74 Bar J Observatory, New Smyr 0 56 38 83 -14 -68 * 25% Old Town FL Bill Helms -70 25% RIDGELAN MS SPEER-LYELL Ob.c/oD 0 54 51 85 -6 -72 24% 831 Rosemary Hill Observatory, 0 56 21 85 -13 -77 22% ALTAMONT FL CentralFLAst.Noeml. 0 56 34 84 -14 -91 15% ** Southern limit plus 1-Sigma ** -93 14% 758 BCC Observatory, Cocoa 0 56 45 83 -15 -100 11% H47 Vicksburg 0 54 45 85 -5 -105 10% ORLANDO FL Laurent Pellerin 0 56 39 84 -14 -109 9% INDIAN H FL HAROLD R. POVENMIRE 0 56 48 83 -15 -143 2% ** Southern limit plus 2-Sigma ** David Dunham, 2007 April 16, 7 pm EDT Phones home 301-474-4722; office 240-228-5609; cell 301-526-5590 e-mail david.dunham@jhuapl.edu home e-mail: dunham@starpower.net . ______________________________________ So far, the planned coverage for Tues. evening's occultation of iota Cancri by (411) Xanthe is very good across the predicted path. But that does not ensure that the event will be well-observed. In fact, if we have a 1.8-sigma shift like we had for the Fortuna occultation last week, or the similar shift that we had for the other bright event, Palma in January, we would end up with NO observations of the Xanthe occultation with the current planned coverage. We now have several observers for the Xanthe occultation, most of them mobile. We need to spread them out more to ensure a successful outcome. Nobody should think that they will really have an occultation. Even at the central line, the chances for an event are formally 53%. Instead, we should spread out enough to give a high probability of success even if the path shifts at least 1-sigma (52 km) north or south; if you are one of the lucky ones to actually have an occultation, that's icing on the cake. When I set up a remote station at Carson, VA last week for the Fortuna occultation, I thought an event there was guaranteed; the formal chances were 99%. But guess what - a miss occurred there, I got the 1% due to the 1.8- sigma south shift that occurred (I did succeed at the other two sites that I ran farther south). So rather than maximize personal chances by crowding near the central line, or even staying all within the predicted path, we should try to achieve a uniform distribution across the whole 1-sigma uncertainty zone (from 91 km north to 91 km south of center), which includes places such as the Jacksonville area to the north and as far south as Rosemary Hill Observatory. Observations from the Jacksonville area would help cover the 1-sigma north zone. I plan to move my own sites to a little inside the 1-sigma limits, to the north with exact location depending on what other observers might do, such as those in Jacksonville who might observe from home (if enough observers cover the 1-sigma northern zone, I might go into the 1-sigma southern zone instead, but maybe mobile observers from central Florida as well as Gainesville might cover that better). Let me know if you are willing to relocate into one of the 1-sigma zones (between the predicted limit and 1-sigma beyond it) and either pick your own location, or ask me for an offset distance there. Remember that you can use the Google maps tab on Derek Breit's Web site to input an offset distance from center, then zoom in on the path to see where that line passes over detailed maps. Although shifts of the actual path from the predicted one should amount to 2 sigma only 2% of the time, we seem to be having shifts of this order more frequently, probably because we are farther from the 1991 mean epoch of the Hipparcos satellite observations, and there are probably systematic errors in addition to the statistical ones affecting the proper motions, which are in part dependent on old ground-based star catalogs. So fixed-site observers at least as far as the 2-sigma limits, such as as far north as Columbus, GA, are encouraged to try this occultation, and even much farther away to check for satellites, as noted in my last message (satellites are a real possibility, as the 6 observers in Japan who timed the occultation by Linus, (22) Kalliope's satellite last November, can attest). For last week's Fortuna occultation, Dan Caton at Dark Sky Observatory in w. N. Carolina was outside the 1-sigma southern limit, but he tried to observe the event, anyway. He had an 11-sec. occultation, half the central duration, providing a valuable southern chord that ensures a good determination of the outline of Fortuna (from it and the other 8 chords farther north). David Very detailed maps for this and all other events listed below are on Derek Breit's interactive Web page that links to the very detailed maps and satellite imagery of maps.google.com with overlays of the occultation paths. Also on Breit's Web site are station lists giving local circumstances, for many observer stations. It gives the predicted time of the event, distance from the updated central line, probability that an occultation will occur there, and altitudes of the star and the Sun. If your station is not in that list, please let me and Derek, breit_ideas@hotmail.com , know so that you can be added to future lists. Also included there now are lists of stars with offset times and declination differences for pre-pointing telescopes for these events.